toc: add GstTocScope and require it in the constructor

toc: add GstTocScope and require it in the constructor

This is because we need to be able to signal different TOCs
to downstream elements such as muxers and the application,
and because we need to send both types as events (because
the sink should post the TOC messages for the app in the
end, just like tag messages are now posted by the sinks),
and hence need to make TOC events multi-sticky.

https://bugzilla.gnome.org/show_bug.cgi?id=678742